Replication fork stalling at sites of abnormal DNA structure is a recognized cause of genomic instability. Increased replication fork stalling (?replication stress?) is a common feature of cancer cells, suggesting that defective processing of the stalled fork is a common mechanism of genomic instability in cancer. The Fanconi Anemia (FA) pathway has a major role in the metabolism and repair of stalled replication forks. FA is a rare, autosomal recessive (or X-linked) disease caused by inactivation of any one of several FA genes. The clinical manifestations of FA include childhood anemia and progressive bone marrow failure, together with short stature and congenital defects affecting a wide variety of organs. The risk of cancer, including solid tumors, is elevated, with particularly high incidence of acute myelogenous leukemia. The gene encoding an early responder of FA pathway, FANCM, is found mutated in some breast cancers. The FA pathway overlaps functionally with the BRCA pathway of hereditary breast/ovarian cancer predisposition?a critical regulator of homologous recombination. The FA pathway is also activated by replication stress, indicating a general role for the FA genes in human cancer and in many other diseases. Thus, deciphering the mechanisms of action of the FA pathway has broad significance for human health. We recently adapted the Escherichia coli Tus/Ter replication fork arrest complex for use in mammalian cells and have used it to quantify both error-free and error-prone homologous recombination induced by a mammalian chromosomal replication fork block. More recently, we identified a novel aberrant repair product of replication fork arrest in mammalian cells, in which small (<10 kb) microhomology-mediated tandem duplications form at the site of replication arrest. FANCM plays a crucial role in suppressing these aberrant repair products at stalled forks. In work proposed here, we will use novel tools, recently developed by the Scully lab, to analyze how FANCM regulates homologous recombination at stalled replication forks. We will identify the mechanisms by which FANCM suppresses tandem duplication at stalled forks. Success in this work will lead to the identification of new targets for therapy in cancer and other human diseases.